1. Field of the Invention
The present invention relates to a reflective mask blank for EUV lithography, a method of manufacturing thereof, a reflective mask for EUV lithography and a method of manufacturing thereof.
2. Description of the Related Art
For an exposure system, the limit of resolution is determined in accordance with a wavelength of light that is irradiated on a semiconductor substrate or the like such as a Si substrate. Then, in such an exposure system, an exposure technique of a transmission optical system using KrF excimer laser whose wavelength is 248 nm, ArF excimer laser whose wavelength is 193 nm or the like is actualized. In such an exposure system, a transmission type mask is used in which a predetermined pattern is formed by a portion that transmits irradiated light and a portion that absorbs the irradiated light, and the predetermined pattern is transferred to a semiconductor substrate or the like by the transmission type mask.
Meanwhile, as an exposure technique using light whose wavelength is shorter than that of the ArF excimer laser, in order to actualize further high resolution of a pattern to be transferred to the semiconductor substrate or the like, an exposure technique using EUV (Extreme Ultra-Violet) light is paid attention. The EUV light means light having a wavelength in a soft X-ray region or in a vacuum ultraviolet region, and specifically, the EUV light means light with a wavelength of about 10 nm to 20 nm, in particular, about 13.2 nm to 13.8 nm (13.5 nm±0.3 nm).
Due to the characteristics of the EUV light, the transmission optical exposure system using a KrF excimer laser light source, an ArF excimer laser light source or the like cannot be adapted to the exposure technique using the EUV light, and thus, a reflective optical exposure system is adapted. Then, in such a reflective optical exposure system, a reflective mask for EUV lithography or a reflective mirror for EUV is used. Here, in this specification, the reflective mask for EUV lithography is referred to as an “EUV mask” as well.
A reflective mask blank for EUV lithography is a patterned precursor of the EUV mask and includes a layer that reflects the EUV light. Here, in this specification, the reflective mask blank for EUV lithography is referred to as an “EUV mask blank” as well. Specifically, the EUV mask blank includes at least a reflective layer that reflects the EUV light on a flat substrate such as glass or the like, and an absorber layer that absorbs the EUV light on the reflective layer. Further, the EUV mask has a structure in which a predetermined pattern is formed at the absorber layer of the EUV mask blank, and is capable of transferring the predetermined pattern to a semiconductor substrate or the like by absorbing the irradiated EUV light at a portion where the absorber layer exists and reflecting the irradiated EUV light at a portion where the absorber layer does not exist.
Generally, a multilayer reflective film is used for the reflective layer in which a high reflective layer having high refractive index to the EUV light and a low reflective layer having low refractive index to the EUV light are alternately stacked. For the multilayer reflective film, for example, a Mo/Si multilayer reflective film including a silicon (Si) layer as a high reflective layer and a molybdenum (Mo) layer as a low reflective layer may be used, and reflectance of the EUV light greater than or equal to 60% can be actualized. Further, for the absorber layer, a material having low reflectance of the EUV light, in other words, a material having a high absorption coefficient of the EUV light is used; for example, a material containing Ta or Cr as a main component may be used.
Here, if defects (contaminant, flaw or pit, for example) exist at a surface of the substrate such as glass or the like, or contaminant is mixed while forming the multilayer reflective film, a periodic structure of the multilayer reflective film is disturbed and defects (a so-called phase defect) are generated in the multilayer reflective film. When such defects are generated, a problem may occur that a pattern on the EUV mask is not correctly transferred to a semiconductor wafer. However, actually, it is technically extremely difficult to eliminate all of the defects of the multilayer reflective film (see non-Patent Document 1, for example).
Thus, a technique has been studied in which positions of the defects in the EUV mask blank are detected, and when manufacturing an EUV mask, a position or direction of the pattern of the absorber layer is adjusted to avoid these defects. Specifically, it is described that a fiducial mark is formed at the reflective layer of the EUV mask blank (see non-Patent Document 2, for example).
Further, the fiducial mark is formed in a concave form or a convex form at an outside area from an actual mask pattern area (an area of 132 mm×132 mm, for example), on the multilayer reflective film, on the substrate or on the absorber layer. Here, for the fiducial mark, typically, a cruciform mark in a plan view is used, and a position of intersection point of the cross of the fiducial mark is often assumed as a fiducial position. Then, information for specifying positions (coordinates) of defects that exist in the actual mask pattern area is obtained based on the fiducial position. Here, coordinates of a plane can be specified when there are at least three fiducial marks at the outside area from the mask pattern area, and the fiducial marks are provided at positions such that intersection points of the crosses of the fiducial marks are not on the same virtual line. Typically, the cruciform fiducial marks are often formed at three corners or four corners outside of the mask pattern area.
FIG. 16 is a plan view schematically illustrating an EUV mask blank 100 with fiducial marks in which fiducial marks 101, 102, 103 and 104 are provided at four corners of an area outside a mask pattern area 105. Then, for the position of each of the fiducial marks, an intersection point of the cross, which is a fiducial position, can be accurately specified by scanning with an electron beam or an ultraviolet light in the vicinity of the fiducial mark with a predetermined space and detecting the difference between reflection intensities, or scattered light intensity generated by concave and convex parts of the fiducial marks. Here, the ultraviolet light in this specification means light with a wavelength range of 170 nm to 400 nm.
As such, as the fiducial mark is the fiducial position for accurately detecting the coordinates of the position of a defect in the EUV mask blank, it is important to specify the fiducial position with high accuracy in an apparatus that is used in manufacturing the EUV mask blank and the EUV mask (in an inspection step, for example). Further, for a method of detecting the fiducial mark, a method or the like is reported in which the fiducial mark is detected even after a resist film is formed on the EUV mask blank by scanning with an electron beam in the vicinity of the fiducial mark from an upper side of the resist film in the step of manufacturing the EUV mask (see Patent Document 1, for example).